In a fraction collection system, a user often wants to collect a compound that has eluted at a particular time. To collect this compound properly, the data system controlling the fraction collector must have an accurate estimate of the fluidic flow rate at the fraction collector inlet. Accordingly, the data system needs to know the amount of time taken by the fluid to reach the fraction collector from the detector (where the compound of interest has been detected). This time, known as the “delay time,” depends on the volume of the tubing connecting the detector outlet to the fraction collector inlet, in addition to the volumetric flow rate of the fluid itself. Although the tubing volume may be known, if the volumetric flow rate of the fluid is not well known, the result may be an improperly estimated delay time. An inaccurate delay time can detrimentally affect the ability of the user to recover the compound of interest completely.
Most devices for measuring fluidic flow rate require some knowledge of at least one property (specific gravity, viscosity, specific heat, etc.) of the fluid, which hinders their usability in a general setting where these fluid properties are not always known. For example, many commercially available velocimeters require calibration for the fluid being measured. Laminar flow meters, on the other hand, require knowledge of the viscosity of the fluid, whereas thermal flow meters require knowledge of the specific heat of the fluid. If a user changes the type of fluid being measured, the instrument requires recalibration
Moreover, in a simple chromatographic system without flow splitting, the fluid follows a single fluidic flow path through the system; the fluid flows from the pump, through the injector, column, and detector, to reach the fraction collector. In this instance, the fluid flow rate is typically estimated to be equal to the flow rate supplied by the pump (which is typically a constant volumetric flow rate, not constant pressure). However, when the pump meters the fluid, this typically occurs at a high pressure, whereas when the fluid reaches the fraction collector, it is near atmospheric pressure. Because of the pressure drop, the fluid expands, and the volumetric flow rate at the fraction collector is higher than at the pump.
Similarly, the temperature of the fluid metered out by the pump may be different when the fluid exits the pump from its temperature upon reaching the fraction collector. In this case, the fluid may expand or contract due to the temperature difference, affecting the overall volumetric flow rate at the fraction collector. Likewise, if the pump is mixing multiple fluids together, the solvent mixture may undergo volume contraction, which can also affect the net volumetric flow rate of the fluid as it reaches the fraction collector.
Instead of a single flow path, a chromatographic system can have multiple paths because of the use of multiple detectors and flow splitting. In such a chromatographic system, the fluid flow rate as the fluid reaches the fraction collector is generally not the same as the flow rate set by the pump.